Prospect first Cub to go deep in inaugural AB since Castro, who also did it in Cincy

"[Ramirez said], 'Everything you've been doing here in Triple-A, do it over there. Don't change anything,'" Soler said. "He knows how hard it is the first day and said, 'Relax, and everything will be all right.'"

Did Ramirez think Soler was ready for the Major Leagues?

"Yes," Soler said, smiling.

Apparently, he was. Soler homered in his first at-bat in the second inning of the Cubs' 7-5 loss to the Reds, hitting a 423-foot blast to center off starter Mat Latos. He's the first Cubs player to hit a home run in his first at-bat since Starlin Castro did so May 7, 2010, also at Great American Ball Park.

Soler's blast was the second in a back-to-back effort with Luis Valbuena that had given the Cubs a 2-0 lead at the time. Soler added an RBI single in a three-run eighth inning to cap a 2-for-4 debut.

"I feel real proud about it," Soler said. "All of my family was watching the game, especially my father here at the game. I feel real happy and proud that I did well today."

The Cubs felt Soler was ready, promoting the 22-year-old outfielder with 30 games remaining. Soler started in right field Wednesday and batted fifth. He is the eighth Cubs player to make his Major League debut this season, joining Arismendy Alcantara, Matt Szczur, Javier Baez, Dallas Beeler, Kyle Hendricks, Neil Ramirez and Tsuyoshi Wada.

"The key to the decision on Soler was the fact that he was going to be a September callup for us mainly because he needs the at-bats," Cubs president of baseball operations Theo Epstein said Wednesday. "He missed significant time because of the hamstring injury. He needs to play, he needs to get the at-bats.

"We've been certain in our minds for a while now that he was going to be a September callup for us. It's the best place for him to continue to get at-bats, continue to learn, continue to make adjustments."

Epstein said they were waiting for a "developmental moment," and it happened when Soler had a mini slump but was able to get back on track. The powerful right-handed hitter hasn't had many slumps this season. He was batting .282 with eight home runs and 29 RBIs with Iowa, including a .373 average with runners on base.

Ranked by MLB.com as the Cubs' No. 5 prospect, Soler belted a three-run homer in the third inning Monday in Iowa's game against Tacoma and was then pulled from the game after two innings. Iowa manager Marty Pevey wouldn't explain why until after the game was over.

"[Pevey] said, 'You don't play any more here,' and they didn't tell me for one hour that I was getting the call," Soler said through coach/interpreter Franklin Font. "I was really surprised."

Soler began the season with Double-A Tennessee but suffered a leg injury after his first game April 3. He went on the disabled list and returned in May, but he again played sporadically. He rehabbed at the Cubs' complex in Mesa, Ariz., and the focus was not just on getting his legs healthy but taking a holistic approach.

The Cubs changed Soler's diet -- more salads, water and less soda -- and worked on his posture. Epstein said Soler had a disproportionate amount of muscle mass located on the anterior side of his body, which was putting extra strain on his hamstrings. The training staff worked to redistribute that muscle mass to make him more balanced. They changed the way Soler runs, too.

Asked if he feels healthy, Soler smiled again and said yes.

After rehabbing in Mesa, he rejoined the Smokies in July and batted .463 in 15 games before he was promoted to Triple-A. In nine games in July with Iowa, he hit .304 and was batting .271 this month.

Epstein said Soler was "born with a very advanced approach at the plate," and that he's made strides with his swing mechanics and swing path so he can get the barrel to the ball better.

"He's always hit the ball hard, he's always controlled the zone, but now he's hitting the ball hard with loft and elevation," Epstein said. "His ground balls have become line drives, his line drives have become fly balls, and his fly balls tend to leave the ballpark. He's a really dangerous hitter. When he's right, he can use the whole field and loft the ball with ease."

That doesn't mean Soler is a finished product.

"He comes to the big leagues with a lot of momentum, but with an awful lot to learn and adjustments to make up here as well," Epstein said.

And Soler knows that. He's already been told about how Major League pitchers are more consistent in terms of their location. He has to be disciplined at the plate.

The Cuban outfielder signed a nine-year, $30 million contract in June 2012 and is on the Cubs' 40-man roster. He's part of the so-called "core four" that also includes Baez, Kris Bryant and Albert Almora. Bryant is at Iowa and leads all Minor League hitters with 43 home runs, while Almora is playing for Tennessee.

Epstein said having Soler join the team on the road will give him time to bond with his teammates. It worked well with Baez, who was promoted when the Cubs were in Denver on Aug. 5.

September can be a tough month to evaluate players, but the Cubs will be facing primarily contending teams. Does a good showing mean Cubs fans can write Soler's name into the 2015 Opening Day lineup?

"It's way too early to answer that," Epstein said. "It depends on a lot of factors. These kids are up here to continue to learn, to continue their development, but also to get opportunities that they can help us win baseball games. That's what this is all about."

Soler's promotion drew cheers from White Sox slugger Jose Abreu.

"Very happy, very happy that he's made it to the Major Leagues and he's able to accomplish one of his dreams, which is to play in the big leagues," Abreu said Tuesday through interpreter and coach Lino Diaz. "My advice to him would be to be very mentally tough and to prepare himself every day to play the game the way he's capable of playing it."

"I've waited two years for this moment," Soler said. "I'll just do everything I can."

Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. MLB.com reporter Scott Merkin contributed to this report. This story was not subject to the approval of Major League Baseball or its clubs.